I put some more thought into a non-casual thread last night, and came up with an idea for a (slightly) plot-driven thread: Character Quest.
Basically, it would involve some person (troper character or otherwise) sending out a request for adventurers to find some artifact, or perhaps somebody's character finding a map or something and finding out where it leads. The starting and ending (should the thread get that far) would be more or less set, but there would be a lot of freedom between those two points to come up with subplots and whatnot. I also think that entering and leaving would be fairly simple; have the leaving character throw in the towel and walk off, or have the new character stumble upon the merry band and wind up joining.
I'm not sure if this idea has been proposed before, and it might not work out so well in practice should we decide to go with it. I do think it matches up with what a lot of people have been suggesting (it's a bit of an excuse plot, the characters in the band have a reason to get to know one another, etc.) and it could provide a plot that isn't "How did we get here?"
Just trying to spark up discussion on the next non-casual thread, since it seems only a few people are willing to say something about it for whatever reason.